Vibration damping refers to the reduction of mechanical vibrations, which can adversely affect a structure. Structures such as bridges, buildings, nuclear reactors, robots, rotating machinery, micromachines, optics and microelectronics benefit from vibration damping, which enhances safety, user comfort, performance, reliability, durability, seismic resistance and blast resistance.
Vibration damping is related to sound attenuation, since a sound wave is a form of vibrational wave. Therefore, materials that are effective for vibration damping tend to be effective for sound attenuation as well.
Vibration damping can be achieved passively or actively. Active damping involves the use of a coordinated set of sensor and actuator, so that the actuator suppresses the vibration through force application in real time as the vibration sensed by the sensor occurs. Due to the sensor and actuator, active damping is expensive. However, it is highly effective. A much less expensive and much more common method of damping is passive. In passive damping, materials that are inherently effective for damping are utilized for dissipating the energy associated with the vibration; sensors and actuators are not used. This invention relates to passive damping.
The damping ability of a material is described by (i) the loss tangent (also known as the loss factor, equal to two times the damping ratio), which describes the ability for oscillatory decay and (ii) the loss modulus (the storage modulus times the loss tangent), which describes the energy dissipation ability. The storage modulus is the elastic modulus during dynamic loading. Both quantities need to high for effective damping.
From the viewpoint of damping performance, damping materials of the prior art fall into three categories: (i) materials exhibiting high loss modulus but low loss tangent, such as cast iron (U.S. Pat. No. 4,966,636), Fe—Mn—Al—C based alloys (U.S. Pat. No. 4,966,636) and continuous carbon fiber reinforced epoxy (U.S. Pat. No. 4,072,084), (ii) materials exhibiting high loss tangent but low loss modulus, such as rubber, silicone, polyurethane, optionally reinforced with short fibers (U.S. Pat. No. 5,916,954; W. Fu and D. D. L. Chung, “Vibration reduction ability of polymers, particularly polymethylmethacrylate and polytetrafluoroethylene”, Polymers and Polymer Composites 9(6):423-426 (2001)), and (iii) materials exhibiting low values of both the loss tangent and the loss modulus, such as cement-based materials containing admixtures (e.g., silica fume, latex and methylcellulose) (X. Fu and D. D. L. Chung, Vibration damping admixtures for cement. Cement and Concrete Research 26(1), 69-75 (1996); X. Fu, X. Li and D. D. L. Chung, Improving the vibration damping capacity of cement. Journal of Materials Science 33, 3601-3605 (1998)).
From the viewpoint of the material composition, damping materials of the prior art fall into five categories: (i) metals, such as metal alloys and metal-matrix composites (U.S. Pat. Nos. 4,966,636, 7,126,257, US2007/0138917), (ii) polymers (U.S. Pat. No. 6,503,974), asphalt (U.S. Pat. No. 5,324,758), their foams (U.S. Pat. No. 6,110,985) and their composites (U.S. Pat. No. 4,623,586, US 2004/0072938, U.S. Pat. Nos. 4,774,954, 5,337,288, 7,029,598), (iii) graphite and graphite-based and carbon-based materials (U.S. Pat. No. 5,143,184), (iv) ceramics (U.S. Pat. No. 7,198,858), and (v) cement-based materials (X. Fu and D. D. L. Chung, Vibration damping admixtures for cement. Cement and Concrete Research 26(1), 69-75 (1996); X. Fu, X. Li and D. D. L. Chung, Improving the vibration damping capacity of cement. Journal of Materials Science 33, 3601-3605 (1998)).
Numerous patents teach about damping materials without giving adequate damping performance data (U.S. Pat. Nos. 5,143,184, 4,966,636, 4,072,084, 5,916,954, 6,503,974, 5,324,758, 6,110,985, US 2004/0072938, U.S. Pat. Nos. 4,774,954 and 5,337,288, 7,029,598, 7,198,858, 7,126,257, US 2007/0138917). Many of these patents report the loss tangent without reporting the loss modulus or the storage modulus.
Cement is a silicate material that cures in the presence of water through a hydration reaction that forms a hydrate. During curing, the cement sets and hardens. Cement particles bond to each other in the presence of water due to this reaction. This type of bonding is known as cementitious bonding.
Cement-based materials are widely used for construction. Thus, cement-based materials with improved vibration damping ability are needed. By using silica fume as an admixture, the damping ability of cement-based materials is increased, so that both the loss modulus and the loss tangent are increased (X. Fu and D. D. L. Chung, Vibration damping admixtures for cement. Cement and Concrete Research 26(1), 69-75 (1996)) (Table 1).
In contrast to the low loss tangent of these cement-based materials is the high value of rubber (Table 1) (W. Fu and D. D. L. Chung, Vibration reduction ability of polymers, particularly polymethylmethacrylate and polytetrafluoroethylene. Polymers and Polymer Composites 9(6), 423-426 (2001)), which, however, suffers from a low loss modulus.
On the other hand, metal-matrix composites have high values of the loss modulus (D. D. L. Chung, Materials for vibration damping. J. Mater. Sci. 36(24), 5733-5738 (2001)) (Table 1), but they suffer from low values of the loss tangent. J. San Juan, M. L. No and C. A. Schuh (Nanoscale shape-memory alloys for ultrahigh mechanical damping. Nature Nanotechnology 4(7), 415-419 (2009)) compare the damping behavior of a large number of polymers, metal alloys, intermetallic compounds, shape-memory alloys, metal-matrix composites and ceramics, and report the best performance (loss tangent 0.196 and elastic modulus 22.6 GPa, i.e. loss modulus 4.43 GPa) for a nanoscale Cu—Al—Ni shape-memory alloy.
San Juan et al. (2009) use a damping figure of merit that is defined as the product of the loss tangent and the square root of the elastic modulus. The best performance material of Juan et al. (a nanoscale Cu—Al—Ni shape-memory alloy) exhibits the figure of merit 0.93 GPa1/2.
Exfoliated graphite is an expanded form of graphite. It is obtained from graphite that has been intercalated. The graphite prior to exfoliation is commonly in the form of flakes, but it can be in other forms, such as fibers.
Graphite has a crystal structure that is layered. The carbon atoms are strongly bonded in the plane of each carbon layer, due to covalent bonding and metallic bonding. However, the carbon atoms are weakly bonded in the direction perpendicular to the carbon layers, due to the bonding being merely due to Van der Waals forces in this direction. As a result of the chemical bonding, graphite is very anisotropic in its properties.
Intercalation refers to the reaction of graphite with a reactant, which thereby enters the graphite and resides between the carbon layers in the graphite. The consequence is a compound known as a graphite intercalation compound. The reactant is known as the intercalate.
The expansion associated with the exfoliation of graphite tends to be mainly along the direction perpendicular to the carbon layers in graphite. The expansion, is commonly by hundreds of times. Due to its shape, the exfoliated graphite obtained from a graphite flake is often known as a worm, with the axis of the worm mainly along the direction perpendicular to the carbon layers. A worm commonly exhibits an accordion morphology, i.e., a morphology in which the carbon layers in the graphite are interconnected like the bellows of an accordion.
The process of exfoliation commonly involves subjecting intercalated graphite to rapid heating (D. D. L. Chung, “Exfoliation of graphite”, Journal of Materials Science 22(12), 4190-98 (1987)). During exfoliation, shear occurs between the carbon layers, thereby enabling the formation of an accordion morphology.
Compression of exfoliated graphite without a binder results in a sheet known as flexible graphite, which is also known by its former tradename “Grafoil”. “Flexible graphite” is a technical term. It does not simply mean graphite that is flexible. (D. D. L. Chung, “Flexible graphite for gasketing, adsorption, electromagnetic interference shielding, vibration damping, electrochemical applications, and stress sensing”, Journal of Materials Engineering and Performance 9(2), 161-163 (2000); X. Luo and D. D. L. Chung, “Vibration damping using flexible graphite”, Carbon 38(10), 1510-1512 (2000)).
TABLE 1Damping properties of materials of the prior artStorageLossFigure ofmodulusLossmodulusmeritMaterial(GPa)tangent(GPa)(GPa1/2)†Neoprene rubbera*0.007450.670.00670.058Polymethylmethacrylatea*3.630.0930.3360.18Flexible graphiteb*1.00.190.210.19Cement paste (plain)cd*1.910.0350.0670.048Cement paste with4.120.0730.3010.15methylcellulose (0.4%#)cd*Cement paste with4.530.1040.4710.22methylcellulose (0.8%#)d*Cement paste with latex2.750.1220.3360.20(20%#)cd*Cement paste with latex3.120.1420.4430.25(30%#)d*Cement paste with5.760.1070.6160.26silica fume(15%#)d*Cement paste with6.200.1050.6510.26silica fume(15%#) andmethylcellulose(0.4%#)cd*Zn—Al matrix SiC whisker990.0323.00.32compositee*Flake reinforced polymerf2.50.410.63Tungsten (95%) with1610.058.10.63In—SngNanoscale Cu—Al—Ni22.60.1964.430.93shape-memory alloyg*Dynamic flexural properties obtained under three-point bending at 0.2 Hz, all obtained in the same laboratory using the same set-up as the data presented in this disclosure for the invented materials.†Product of the loss tangent and the square root of the elastic modulus.#% per mass of cement.aW. Fu, and D. D. L. Chung, “Vibration reduction ability of polymers, particularly polymethylmethacrylate and polytetrafluoroethylene”, Polymers and Polymer Composites 9(6), 423-426 (2001).bX. Luo and D. D. L. Chung, “Vibration damping using flexible graphite”, Carbon 38(10), 1510-1512 (2000).cX. Fu and D. D. L. Chung, “Vibration damping admixtures for cement”, Cement and Concrete Research 26(1), 69-75 (1996).dX. Fu, X. Li and D. D. L. Chung, “Improving the vibration damping capacity of cement”, Journal of Materials Science 33, 3601-3605 (1998).eD. D. L. Chung, “Materials for vibration damping”, Journal of Materials Science 36(24), 5733-5738 (2001).fU.S. Pat. No. 4,623,586, with the highest values (which occur at about −20° C.) shown in Table 1 (1010 dyne/cm2 = 1 GPa).gJ. San Juan, M. L. No and C. A. Schuh, “Nanoscale shape-memory alloys for ultrahigh mechanical damping”, Nature Nanotechnology 4(7), 415-419 (2009).
The formation of a flexible graphite sheet in the absence of a binder is due to the mechanical connection between the physical units of exfoliated graphite. A worm is an example of a physical unit of exfoliated graphite. In case of exfoliated graphite that exhibits an accordion morphology, mechanical connection between the physical units of exfoliated graphite is enabled by the accordion morphology, since the edge region of a carbon layer of one accordion (i.e., one unit) fits between the carbon layers in the edge region of the adjacent accordion (i.e., the adjacent unit). This mechanical connection is akin to fastening, but it is in a microscopic scale (a scale ranging from the nanoscale to the microscale). Upon compression, the mechanical connection becomes tight and mechanical interlocking is achieved between the physical units of exfoliated graphite.
Compared to other materials, flexible graphite is moderately attractive for damping (Luo and Chung, 2000) (Table 1). Its loss tangent is much lower than that of rubber, though its figure of merit is higher than that of rubber (Table 1). However, flexible graphite is attractive for its chemical inertness, low coefficient of thermal expansion and substantial thermal conductivity. It is used as an asbestos replacement and as a macroscopic insert in a structure for damping enhancement (U.S. Pat. No. 5,143,184).
Cement-graphite composites of the prior art involve either graphite flakes (not exfoliated) or carbon fibers (not exfoliated) used as admixtures for tailoring the thermoelectric behavior (S. Wen and D. D. L. Chung, “Thermoelectric behavior of carbon-cement composites”, Carbon 40, 2495-2505 (2002); V. H. Guerrero, S. Wang, S. Wen and D. D. L. Chung, “Thermoelectric property tailoring by composite engineering”, Journal of Materials Science 37(19), 4127-4136 (2002)), electromagnetic shielding behavior (S. Bhattacharya, V. K. Sachdev, R. Chatterjee, R. P. Tandon, “Decisive properties of graphite-filled cement composites for device application”, Applied Physics A 92, 417-420 (2008) or the electrochemical behavior (F. Peinado, A. Roig and F. Vicente, “Electrochemical characterization of cement/graphite and cement/aluminium materials”, Journal of Materials Science Letters 13, 609-612 (1994)).
In general, the loss tangent, elastic modulus and loss modulus are properties that vary with temperature for a given material. For example, for a thermoplastic polymer, softening upon heating increases the loss tangent but decreases the elastic modulus. Thus, the loss tangent is relatively high, but the elastic modulus is relatively low after softening; whereas the loss modulus is relatively high, but the loss tangent is relatively low before softening.
In terms of the figure of merit (defined as the product of the loss tangent and the square root of the elastic modulus) (Table 1), the highest performance damping material of the prior art is a nanoscale Cu—Al—Ni shape-memory alloy (J. San Juan, M. L. No and C. A. Schuh, “Nanoscale shape-memory alloys for ultrahigh mechanical damping”, Nature Nanotechnology 4(7), 415-419 (2009)), but this material suffers from a low value of the loss tangent. In terms of the figure of merit (Table 1), the second highest performance damping materials of the prior art are tungsten with In—Sn (San Juan et al., 2009) and a flake reinforced polymer (U.S. Pat. No. 4,623,586), but the tungsten composite suffers from a low value of the loss tangent and the flake reinforced polymer suffers from a low value of the loss modulus.
Cement-based materials are commonly modified by the use of admixtures. An admixture is an additive that is introduced by mixing with the other ingredients in the cement mix. Admixture have been used in the prior art to improve the damping ability of cement-based materials.
Admixtures are commonly used at a minor proportion, so that the physical units (e.g., particles) of an admixture do not touch one another adequately to form a continuous physical network in the resulting cement-based material.
By using an admixture at a sufficiently high volume fraction in a cement-based material, the physical units (e.g., particles or short fibers) of the admixture may touch one another, thereby forming a physically continuous path. This physical continuity associated with the touching is shown by a low electrical resistivity in the resulting cement-based material in case that the admixture is electrically conductive. However, electrical connectivity is to be distinguished from mechanical connectivity. In spite of the touching, the physical units of the admixture are not mechanically connected. In other words, the physical units of the physically continuous network are not mechanically connected and mechanical connectivity is absent.
Among cement-based materials of the prior art, the material that exhibits the highest damping figure of merit (Table 1) is cement paste with admixtures in the form of silica fume and methylcellulose (X. Fu and D. D. L. Chung, “Vibration damping admixtures for cement”, Cement and Concrete Research 26(1), 69-75 (1996)). The silica fume is in the amount of 15% by mass of cement, whereas the methylcellulose is in the amount of 0.4% by mass of cement. Silica fume is effective for improving the damping ability due to the small size of the silica fume particles and the consequent large area of the interface between silica and cement in the resulting composite. The slight slippage at the interface during vibrating provides a mechanism for damping. However, this cement-based material suffers from low values of both loss modulus and loss tangent (Table 1).
Latex particles used in the prior art as an admixture in cement-based materials for damping improvement is an elastomer (akin to rubber). Latex improves the damping ability of cement, due to the viscoelastic nature of latex. However, the improvement is by a limited degree, so that both the loss modulus and the loss tangent remain low, even when the latex is present in the large amount of 30% by mass of cement (Table 1).
The introduction of methylcellulose (a water soluble polymer) to the cement mix when the methylcellulose has been dissolved in water promotes uniform distribution of the methylcellulose in the resulting composite. Thus, methylcellulose is able to improve the damping ability of a cement-based material even when it is used at a very low proportion, such as 0.4% by mass of cement (Table 1). The mechanism of damping improvement due to methylcellulose is associated with the viscoelastic nature of methylcellulose. The damping improvement due to methylcellulose is by a degree that is comparable to or below that attained by latex (Table 1). This means that both the loss modulus and the loss tangent remain low after introduction of methylcellulose.
Rubber (as that from used tires) is used as an aggregate in concrete to improve the abilities for vibration damping, sound absorption and impact resistance (US 2005/0096412). However, the rubber aggregate suffers from its inadequate bonding with the cement matrix.
The present invention is directed to overcoming these and other deficiencies in the art.